When a steam generator is being cold started or is operating at low load ("load" here being used to mean the degree to which a boiler is being used relative to its capabilities) solid fuel powders are ignited by flames from high-grade fuels such as liquid or gaseous hydrocarbons. Initial ignition can be provided by diesel oil for example, followed by fuel oil and then the main solid fuel. This assistance may last up to three hours depending on the quality of the main solid fuel. This is the time necessary to reach a load of about of 35% to 40% which enables suitable stability flame to be obtained without assistance from high-grade fuel.
Further, assistance at low load is now being required more frequently than in the past, since fossil fuel boilers are being used more and more, not as base units but as auxiliary units, to powerful units which burn nuclear fuels and in which there is very little possibility of varying the load.
Therefore, the lower the grade of the main solid fuel, the higher the consumption of these auxiliary fuels (diesel oil light or heavy fuel oil, natural gas). This has a great incidence on running cost when it is necessary to import these fuels.
The main components of apparatus which equips a conventional installation are: a main combustion chamber; a pilot burner; a liquid fuel storage tank; and distribution pumps and storage cocks to bring the liquid fuel to a pilot igniter which forms part of the pilot burner. The pilot burner is disposed upstream from the main combustion chamber and the pilot igniter lies generally on the axis of the pilot burner which has a peripheral ring surrounding the pilot igniter to supply a mixture of solid fuel powder and so-called primary air under pressure to the main combustion chamber. So-called secondary air, also supplied under pressure, is provided at the combustion chamber.
One common particular prior art arrangement uses a liquid fuel which is particularly rich in calories for auxiliary ignition thus requiring only one pilot igniter of relatively small volume and capable of being disposed in a conventional configuration. Likewise, the combustion air is used only in the combustion chamber since combustion is thus sufficiently assisted.
Preferred embodiments of the apparatus in accordance with the present invention provide ignition and combustion support for solid fuel powders without using a high-grade auxiliary fuel which is expensive and sometimes difficult to obtain.